Fuel dispenser flow meter having vapor pressure correction arrangement

ABSTRACT

A meter for measuring the flow rate of fuel in a fuel dispenser comprises a housing with an axial bore extending therethrough and defining an inlet and an outlet. A flow indicating mechanism comprising one or more rotatable components are mounted in the axial bore of the housing and configured to rotate as fuel flows from the inlet to the outlet, wherein each rotation indicates fuel flow. A pulse generating device senses rotation of the rotatable components and generates an output signal indicative of the fuel flow. Finally, a vapor generating device is configured to generate vapor bubbles that flow through the flow indicating mechanism.

PRIORITY CLAIM

This application is based upon and claims priority to provisional application Ser. No. 62/073,450, filed Oct. 31, 2014. The foregoing application is incorporated fully herein by reference for all purposes.

FIELD OF THE INVENTION

Embodiments of the present invention relate generally to flow meters for use in fuel dispensers. More specifically, embodiments of the present invention relate to a mechanical arrangement for correction of vapor pressure error that can occur in some situations.

BACKGROUND

In a retail service station environment, customers use a fuel dispenser to pump fuel from an underground storage tank (UST) into their vehicle's fuel tank. Fuel flow meters are used by the fuel dispenser to measure the amount of fuel being dispensed. As fuel is dispensed, it flows through the flow meter, which generates an output signal proportional to the flow rate or volume of the fuel. The output signal, generally in the form of a pulse train, is received by the fuel dispenser control system, which calculates and displays the volume and price of fuel dispensed.

Fuel flow meters must be very accurate in order to ensure that customers are properly charged for the fuel that has been dispensed. Seasonal variations in fuel temperature can cause errors in flow meter output, thus requiring frequent meter testing and recalibration to ensure accuracy. Fuel flow meters must comply with regulatory requirements regarding measurement accuracy which are enforced by Weights & Measures (or an equivalent regulatory authority). Failure to comply with these regulatory requirements can result in an entire fueling station being temporarily shut down until the issues with the flow meters are rectified.

SUMMARY

Example embodiments of the present invention recognize and address considerations of prior art constructions and methods.

According to one aspect, an example embodiment of the present invention provides a meter for measuring the flow rate of fuel in a fuel dispenser, comprising a housing with an axial bore extending therethrough and defining an inlet and an outlet. A flow indicating mechanism comprising one or more rotatable components are mounted in the axial bore of the housing and configured to rotate as fuel flows from the inlet to the outlet, wherein each rotation indicates fuel flow. A pulse generating device senses rotation of the rotatable components and generates an output signal indicative of the fuel flow. Finally, a vapor generating device is configured to generate vapor bubbles that flow through the flow indicating mechanism.

Another aspect of the present invention provides for a flow indicating mechanism comprising two interlocking helically-shaped spindles. The vapor generating device may be a mesh-like arrangement with multiple sharp-edge orifices, a spring-loaded check valve with sharp vapor-generating edges, or a cavitation machine.

A still further aspect of the present invention provides a flow meter that comprises a bypass passage that routes fuel around a vapor generating mechanism when the fuel is above a certain temperature. The bypass passage may be opened and closed by a valve actuated by a bimetallic temperature-responsive device.

Another example embodiment of the present invention provides a vapor generating device for generating vapor bubbles in fuel flowing through a flow meter. The vapor generating device comprises a support structure configured to be positioned in the fuel flow path upstream of the flow meter's flow indicating mechanism and a permeable structure spanning an opening in the support structure. Vapor bubbles are generated when fuel flows through the permeable structure. The permeable structure may be, for example, a wire mesh screen and may be configured to cover the entire flow meter inlet.

In some example embodiments, the support structure of the vapor generating device is an integrated portion of the flow meter housing. The vapor generating device may also comprise a bypass valve which is configured to divert a portion of the flowing fuel around the permeable structure and into the flow meter when a certain temperature is reached. The bypass valve may be opened and closed, for example, by a bimetallic control device. The bypass valve may comprise a temperature-dependent variable orifice which becomes larger as the fuel temperature increases.

Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of example embodiments in association with the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including the best mode thereof directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which:

FIG. 1 is a diagrammatic view of a fuel dispensing facility where example embodiments of the present invention may be implemented;

FIG. 2 is a diagrammatic view of internal fuel flow components of the fuel dispenser of FIG. 1;

FIG. 3 is a sectional view of a flow meter constructed in accordance with an embodiment of the present invention;

FIG. 4 illustrates a cross-sectional view of the flow meter of FIG. 3 taken along line 4-4;

FIG. 5 is a plot generally showing the relationship between fuel vaporization and fuel temperature;

FIG. 6 illustrates a vapor generating device constructed in accordance with an example embodiment of the present invention;

FIG. 7 is a plot showing the relationship between fuel vaporization and fuel temperature within a flow meter both with and without the presence of a vapor generating device; and

FIG. 8 illustrates an alternative configuration of a vapor generating device constructed in accordance with an example embodiment of the present invention.

Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention.

DETAILED DESCRIPTION

Reference will now be made in detail to certain preferred embodiments of the present invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations.

Some embodiments of the present invention are particularly suitable for use in a fuel dispensing environment, and the below discussion will describe example embodiments in that context. However, those of skill in the art will understand that the present invention is not so limited. In fact, it is contemplated that embodiments of the present invention may be used in other situations where the flow rate of liquid or gas products needs to be measured. In this regard, “flow rate” is used in the description below as a generic term for the rate of fluid dispensing.

FIG. 1 is a schematic diagram of a fuel dispensing facility 10 where exemplary embodiments of the present invention may be implemented. At these facilities, fuel is typically stored in underground storage tanks (USTs) 12 which are in fluid communication with one or more fuel dispensers 14 via underground piping conduit 16. Fuel dispenser 14 may be, for example, the ENCORE® fuel dispenser or the SK700 fuel dispenser, both sold by Gilbarco Veeder-Root. Those of skill in the art will appreciate, however, that the present invention may be used in a variety of fuel dispensers, or other dispensers of liquid or gas products.

Fuel dispenser 14 includes a housing with at least one flexible fuel hose 18 extending therefrom. Fuel hose 18 terminates in a manually-operated fuel dispensing nozzle 20 adapted to be inserted into a fill neck of a vehicle's fuel tank. When a customer places the spout of the fuel dispensing nozzle 20 into the vehicle's fuel tank and pulls the lever, a submersible turbine pump (STP) 22 pumps fuel through the underground fuel piping conduit 16 to the fuel dispenser 14. As will be discussed in more detail below in reference to FIG. 2, various fuel handling components located inside the housing allow fuel to be received from underground piping conduit 16 and delivered through the inner conduit 18 a of coaxial fuel hose 18 and nozzle 20 to a vehicle's tank.

The fuel dispenser 14 has a customer interface 24 which may include a display 26 that shows the amount and price of fuel dispensed. The customer interface 24 may also include additional displays to, for example, provide instructions for basic transaction functions, such as initiating dispensing of fuel. The dispenser also preferably includes a credit card reader and a PIN pad to allow the customer to pay for the fuel at the dispenser using credit or debit cards.

In this embodiment, as the vehicle fuel tank is filled, entering fuel displaces fuel vapor already in the tank. Fuel dispensers are often equipped with vapor recovery systems for capturing these escaping vapors and delivering them back to the ullage space 28 of the UST 12. Exemplary vapor recovery systems may use a vacuum source to draw escaping vapors along a vapor recovery path back into the ullage space 28. The vapor recovery path typically begins at the nozzle spout, where air and vapor are drawn in through a boot or a series of intake ports (not shown). The recovered vapors then flow through the nozzle to the outer vapor recovery conduit 18 b of the fuel hose 18. Inside the fuel dispenser 14, a manifold (not shown) separates conduits 18 a and 18 b and connects them with piping conduit 16 and vapor return pipe 30, respectively. Specifically, conduit 18 a is in fluid communication with underground piping conduit 16. Outer vapor recovery conduit 18 b is in fluid communication with vapor return pipe 30, which returns the recovered vapors back into the ullage space 28 of the UST 12.

FIG. 2 is a schematic illustration of internal components of fuel dispenser 14. In general, fuel may travel from an underground storage tank (UST) via main fuel piping 16 through shear valve 31. As is well known, shear valve 31 is designed to close the fuel flow path in the event of an impact to fuel dispenser 14. U.S. Pat. No. 7,946,309 to Reid et al., hereby incorporated by reference in its entirety for all purposes, discloses an exemplary shear valve adapted for use in service station environments. Shear valve 31 contains an internal fuel flow path to carry fuel from main fuel piping 16 to internal fuel piping 32.

After fuel exits the outlet of the shear valve 31 and enters into the internal fuel piping 32, it may encounter an electromechanical valve 33 positioned upstream of a flow meter 34. However, in some fuel dispensers, valve 33 may be positioned downstream of the flow meter 34. Valve 33 may be, for example, a proportional solenoid controlled valve, as described in U.S. Pat. No. 5,954,080 to Leatherman, hereby incorporated by reference in its entirety for all purposes.

Valve 33 is under control of a control system 35 via a control valve signal line 36. Control system 35 may comprise a microprocessor, microcontroller, or other suitable electronics with associated memory and software programs running thereon. Control system 35 controls the application of power from a power source to the valve coil. In this manner, control system 35 can control the degree of opening and closing of the proportional control valve 33 via the valve coil to allow fuel to flow or not flow through meter 34 and on to hose 18 and nozzle 20 at a desired flow rate, or not to flow at all.

A vapor barrier typically separates some of the mechanical or hydraulic components from the electrical components of the fuel dispenser 14. For example, proportional control valve 33 is typically located below a vapor barrier 38 delimiting a hydraulics compartment 40 of the fuel dispenser 14. Control system 35, on the other hand, is typically located in an electronics compartment 42 of fuel dispenser 14 above vapor barrier 38. In this embodiment, after fuel exits proportional control valve 33, it may flow through meter 34, which measures the volume and/or flow rate of the fuel.

Flow meter 34 may be a positive displacement or inferential flow meter having one or more rotors which rotate on one or more shafts. Some examples of positive displacement flow meter technology which may be used with embodiments of the present invention are provided in U.S. Pat. No. 6,250,151 to Tingleff et al., U.S. Pat. No. 6,397,686 to Taivalkoski et al., and U.S. Pat. No. 5,447,062 to Kopl et al., each of which is hereby incorporated by reference in its entirety for all purposes. Likewise, examples of inferential flow meter technology are provided in U.S. Pat. No. 7,111,520 to Payne et al., U.S. Pat. No. 5,689,071 to Ruffner et al., and U.S. Pat. No. 8,096,446 to Carapelli, each of which is also incorporated by reference herein in their entireties for all purposes.

As shown in FIG. 2, meter 34 is operatively connected to a displacement sensor 44 that generates a signal indicative of the volume or flow rate of fuel and periodically transmits the signal to control system 35 via a signal line 46. In this manner, control system 35 can update the total gallons dispensed and the price of the fuel dispensed on display 26 via a communications line 47. In one embodiment, displacement sensor 44 may be a pulser. Those of ordinary skill in the art are familiar with pulsers that may be utilized with embodiments of the present invention. For example, displacement sensor 44 may be the T18350-G6 pulser offered by Gilbarco Inc. Reference is hereby made to U.S. Pat. No. 8,285,506, entitled “Fuel Dispenser Pulser Arrangement,” granted Oct. 9, 2012, the entire disclosure of which is incorporated by reference herein for all purposes. In other embodiments, however, displacement sensor 44 may be another suitable displacement sensor.

In this embodiment, as fuel leaves flow meter 34, it enters a flow switch 48. Flow switch 48, which preferably includes a one-way check valve that prevents back flow through fuel dispenser 14, provides a flow switch communication signal to control system 35 via the flow switch signal line 50. The flow switch communication signal indicates to control system 35 that fuel is actually flowing in the fuel delivery path and that subsequent signals from sensor 44 are due to actual fuel flow.

After the fuel leaves flow switch 48, it exits through internal fuel piping 32 to be delivered through fuel hose 18 and nozzle 20 for delivery to the customer's vehicle. Nozzle 20 typically includes a manually-actuated valve as is well-known in the art. Further information on and examples of fuel dispensers and retail fueling environments are provided in U.S. Pat. Nos. 6,435,204; 5,956,259; 5,734,851; 6,052,629; 5,689,071; 6,935,191; and 7,289,877, all of which are incorporated herein by reference in their entireties for all purposes.

FIG. 3 illustrates a cross-sectional view of flow meter 34 in which example embodiments of the present invention may be implemented. This type of flow meter is known in the art as an axial positive displacement meter, meaning that each rotation of the spindle captures and displaces a specific volume of fuel. The flow meter 34 is placed inline with the internal fuel piping 32 and comprises a housing 62 with a hollow bore 64 extending axially from an inlet 66 to an outlet 68. Inside the housing 62, two spindles 70 are supported on their own shafts 72 and are oriented in parallel with the hollow bore 64 of the housing 62. Each spindle shaft 72 is supported by a pair of bearings 74, one located upstream and one located downstream of the spindles 70. The flow meter housing 62, spindles 70, shafts 72, bearings 74, and any other components exposed to fuel are preferably constructed of suitable metal or another material that may suitably withstand the corrosive effects of the fuel.

The spindles 70 each comprise one or more interlocking threads such that the spindles 70 rotate together as fuel flows axially through the inlet 66, across the spindles 70, and out of the outlet 68. During each rotation of the spindles 70, a specific volume of liquid is captured from the inlet 66 and the same amount is expelled from the output 68. The flowing fuel causes the spindles 70 to rotate at a speed proportional to the fuel flow rate. Therefore, by measuring the rotational speed of one of the shafts 72, the flow rate of the liquid fuel can be determined.

One method of measuring the rotational speed of the spindles, and thus the rate of fluid flow, is to detect the rotational speed of one of the spindle shafts using a series of magnets and a measuring transducer. In the example embodiment, the magnets 76 are mounted on an exciter disc 78 that is coaxially mounted on the downstream end of one of the spindle shafts 72 (see FIG. 4). The magnets 76 are uniformly spaced around a circumferential portion of the exciter disc 78. A measuring transducer, e.g., a Hall-effect sensor 80, is placed opposite of the exciter disc 78 in a compartment 82 that is separated from the fluid flow. The Hall-effect sensor 80 is configured to generate a pulse train in response to the passing magnets 76. Each pulse may indicate that a specific volume of fuel has been displaced (e.g., 1 pulse=1/1000 gallon of fuel). Therefore, as the spindle shaft 72 spins, the Hall-effect sensor 80 generates a pulse train that is proportional to the rotational speed of the spindle 70 and thus can provide a measure of the flow rate through the meter 34. One skilled in the art will appreciate that this method of measuring the rotational speed of the spindle shafts is only one possible method. Other methods may use, for example, a tachometer or an encoder.

In general, positive displacement meters can provide a measure of the volumetric flow of liquid through the flow meter. However, any vapor generated in the captured volume can cause undesired variation in the measurement. The size of the flow meter components and the captured volume of liquid are small. Therefore, the spindles must be able to rotate at very high speeds in order to meet expected system flow rates. This high speed rotation generates significant internal fluid friction between the rotating components and the housing. The edges of the rotating spindles cause high shear forces and generate localized pressure drops where the edges of the spindles move quickly through the fuel. Therefore, vapor generation is increased, and the presence of vapor bubbles in flow meters is a common occurrence. Typically, the liquid is vaporized in the meter itself, and the vaporized fuel condenses back to liquid form downstream after exiting the flow meter outlet. However, because the flow meter's accuracy depends on the displacement of fluid, the presence of vapor bubbles within the meter introduces error into the volumetric measurement.

In general, the amount of vapor generated in a fluid is directly related to the fluid's equilibrium vapor pressure. When the localized pressure of a fluid is below the fluid's equilibrium vapor pressure, portions of the fluid convert from liquid to vapor bubbles. This is commonly referred to as cavitation. Thus, lower equilibrium vapor pressure results in the generation of less vapor, while more vapor is generated as the fluid's equilibrium vapor pressure increases.

The equilibrium vapor pressure of fuel—and thus its tendency to vaporize—is directly related to the fuel temperature. For example, FIG. 5 shows the relationship between fuel vaporization and fuel temperature generally. As can be seen, as the fuel temperature increases, the fuel has an increased tendency to vaporize. In addition, the tendency of fuel to vaporize is independent of the fluid flow rate, system pressure, and the location of the meter within the hydraulics.

As a result, during the summer when fuel temperatures are elevated, the equilibrium vapor pressure of the fuel is higher, and the generation of vapor bubbles in the flow meter spindles increases. The increased vapor in the meter causes less liquid fuel to be displaced during each rotation of the spindles. By contrast, as the fuel temperature decreases in the winter, the generation of vapor in the flow meter spindles is decreased. Therefore, fuel flow meters can exhibit seasonal variations and require frequent testing and calibration to ensure accurate measurement.

The pulse train generated by the Hall-effect sensor 80 represents the raw or uncorrected flow rate independent of mechanical wear or other factors that require compensation. In order to determine the actual or corrected flow rate, the pulse train may be received by a signal processor, where it may be processed, compensated, corrected, or otherwise analyzed. For example, the signal processor may count the number of pulses and apply a compensation or calibration factor to calculate the corrected flow rate. The control system 35 can then use the corrected flow rate to determine the volume and price of the fuel dispensed, and present this information to the customer via display 26. The signal processor may be packaged with the flow meter, may be a separate stand-alone unit, or may be incorporated into the control system 35. In any event, the compensation applied is based on the most recent calibration of the meter.

Occasional calibration can be effectively used to compensate for relatively static errors. However, due to the temperature-dependent nature of vapor bubble generation, and the resulting seasonal variation in the presence of vapor bubbles, flow meters must be recalibrated multiple times throughout the year to remain accurate. Thus, the amount of vapor generated—and thus the calibration—varies seasonally. If the calibration was conducted during a prior season, it will not likely compensate for the vapor effects discussed above. Thus, failure to properly maintain the calibration of the flow meter may result in a fuel dispenser falling outside regulatory limits regarding the accuracy of the flow meters.

In an example embodiment of the present invention, a mechanical device, referred to herein as a vapor generator 92 is use to minimize the seasonal impact of fuel temperature on flow meter calibration. As shown in FIG. 6, vapor generator 92 in this embodiment is configured as a cone-shaped structure that can be placed in the inlet 66 of the flow meter 34 such that it is inline with and upstream of the flow meter spindles 70 (as shown in FIG. 3). Vapor generator 92 generally comprises a support structure 93 that is covered by a permeable structure or webbing 94 through which fuel may flow. The permeable structure can be any material suitable to withstand the flow of fuel without degradation and is configured to generate vapor bubbles in the fuel flowing through it. In the example embodiment of FIG. 6, the support structure 93 comprises a plastic frame and the permeable structure 94 comprises a steel mesh with a multitude of sharp edge orifices. Vapor bubbles are generated when fluid enters the flow meter housing 62 through the inlet 66 and passes through the vapor generator 92. These vapor bubbles will flow with the fuel through the spindles 70 and become a portion of the captured volume.

It should be appreciated that although the vapor generator 92 of FIG. 6 is a separate device that can be inserted into a flow meter, such a device can also be integral to the housing of the flow meter. An insert device, however, may be sold separately and is ideal for retrofit applications. Alternatively, new flow meters may have the vapor generating device built into the meter housing. Thus, the vapor generator is conducive to use is new flow meters, but is also available for retrofitting flow meters currently in operation.

FIG. 7 shows the relationship between fuel vaporization and fuel temperature for fuel in a flow meter both with and without a vapor generating device. In general, the tendency of the fuel to vaporize is similar to that shown in FIG. 5. However, within the confined volume of a flow meter, the amount of vapor bubbles tends to max out at a certain point. The point at which the amount of vapor bubbles tends to max out is referred to herein as the vapor maximum point (as indicated by the dotted line in FIG. 7). When no vapor generator 92 is present, the vaporization profile constantly increases throughout the typical seasonal temperature range—i.e., between T_(LOW) and T_(HIGH). The vapor maximum point is not reached until the fuel temperature is well beyond the normal operating range. However, when a vapor generator 92 is installed in the flow meter, the presence of vapor bubbles is increased, even at low temperatures. Thus, as can be seen in FIG. 7, when a vapor generator 92 is installed, the flow meter exhibits vapor bubbles even at low temperatures. Further increasing the fuel temperature (e.g., in the summer) does not result in a significant increase in the generation of vapor bubbles. The net effect is that vapor bubbles will be present in the flow meter almost year-round, and the seasonal variation in the presence of vapor bubbles is reduced or eliminated. This results in equivalent meter accuracy for fluids with low and high equilibrium vapor pressure, thus allowing for less-frequent calibration and more accurate year-round performance.

One skilled in the art will appreciate that a conical mesh vapor generator 92 as discussed above with respect to FIG. 6 is only one example of a device for generating vapor. Many other methods and/or devices for vapor generation are also contemplated and within the scope of the invention. For example, the device may be variable in nature such that the amount of vapor generated could change relative to the flow rate. In one embodiment, the vapor generator could be spring-loaded device, such as a specialized check valve, that includes sharp vapor generating edges. In another example embodiment, a cavitation generating machine could be used to generate vapor. The cavitation generating machine could be an active electromechanical device that displaces volume in rapid manner that would increase/decrease the fluid space to impart changes in pressure. As an example, a piezoelectric device acting like a piston pushing into and out of the fluid path could be used. As another example, an oscillating device (such as a small blade) could be inserted into the fluid path that moves rapidly enough within the liquid that vapor is generated. In still another embodiment, a noise generator could generate hydraulic noise—i.e., rapid changes in system pressure—which would cause fluid acceleration and deceleration across the rotating spindles and vaporization even in fluids with a low equilibrium vapor pressure.

Alternatively, as shown in FIG. 8, the vapor generating device may be a variable vapor generator 100. The variable vapor generator can comprise a frame 102 that is configured to fit into the inlet 66 of a flow meter 34. A mesh screen 104 can generate vapor as fuel flows through the inlet 66. The variable vapor generator 100 may also include a bypass passage 106 that routes some or all of the flowing fuel around the mesh screen 104 when a certain fuel temperature is reached. For example, a bimetallic control device could operate as a bypass valve 108 that opens when a certain threshold fuel temperature is reached (similar to a car thermostat). In addition, the bypass passage 106 may have a variable orifice that opens more as the temperature increases. In this manner, employing a bypass valve 108 and bypass passage 106 would reduce vapor generation at higher temperatures.

While one or more example embodiments of the invention have been described above, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. In addition, the embodiments depicted are presented by way of example only and are not intended as limitations upon the present invention. Thus, it should be understood by those of ordinary skill in this art that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the scope and spirit thereof. 

1. A meter for measuring the flow rate of fuel in a fuel dispenser, comprising: a housing with an axial bore extending therethrough and defining an inlet and an outlet; a flow indicating mechanism comprising at least one rotatable component mounted in the axial bore of the housing and configured to rotate as fuel flows from the inlet to the outlet; a pulse generating device configured to sense rotation of the rotatable components and generate an output signal indicative of fuel flow; and a vapor generating device configured to generate vapor bubbles that flow through the flow indicating mechanism.
 2. The fuel flow meter of claim 1, wherein the flow indicating mechanism comprises two interlocking helically-shaped spindles.
 3. The fuel flow meter of claim 1, wherein the vapor generating device comprises a mesh-like arrangement with multiple sharp-edge orifices.
 4. The fuel flow meter of claim 1, wherein the vapor generating device is located in the inlet of the housing.
 5. The fuel flow meter of claim 1, wherein the vapor generating device comprises a spring-loaded check valve with sharp vapor-generating edges.
 6. The fuel flow meter of claim 1, wherein the flow meter comprises a bypass passage that routes fuel around a vapor generating mechanism when the fuel is above a certain temperature.
 7. The fuel flow meter of claim 5, wherein the bypass passage is opened and closed by a valve actuated by a bimetallic temperature-responsive device.
 8. The fuel flow meter of claim 1, wherein the vapor generating device comprises a cavitation machine.
 9. A vapor generating device for generating vapor bubbles in fuel flowing through a flow meter, comprising: a support frame configured to be positioned in the fuel flow path upstream of the flow meter's flow indicating mechanism; and a permeable structure spanning an opening in the support frame, wherein vapor bubbles are generated when fuel flows through the permeable structure.
 10. The vapor generating device of claim 9, wherein the permeable structure comprises a wire mesh screen.
 11. The vapor generating device of claim 9, wherein the permeable structure is configured to cover substantially the entire flow meter inlet.
 12. The vapor generating device of claim 9, wherein the support structure is an integrated portion of a flow meter housing.
 13. The vapor generating device of claim 9, further comprising a bypass valve which is configured to divert a portion of the flowing fuel around the permeable structure and into the flow meter when a certain temperature is reached.
 14. The vapor generating device of claim 13, wherein bypass valve is opened and closed by a bimetallic control device.
 15. The vapor generating device of claim 13, wherein the bypass valve comprises a temperature-dependent variable orifice which becomes larger as the fuel temperature increases.
 16. A method of measuring the flow rate of fuel in a fuel dispenser comprising: providing a fuel flow meter that measures the flow rate of fuel through the meter; placing a vapor generating device configured to generate vapor in flowing fuel upstream of a flow indicating mechanism of the fuel flow mater; and measuring the flow rate through the meter.
 17. A fuel dispenser comprising: a housing; internal piping located in said housing for fluid communication with a source of fuel; a flexible hose external to said housing, a proximal end of said hose being in fluid communication with said internal piping; a nozzle located at a distal end of the flexible hose; and a fuel flow meter for measuring the flow rate of fuel through said internal piping, said fuel flow meter including: a housing with an axial bore extending therethrough; a flow indicating mechanism comprising at least one rotatable component mounted in the axial bore of the housing; and a vapor generating device configured to generate vapor bubbles that flow through the flow indicating mechanism. 